The present invention relates to a shape memory alloy for repeated use and containing no noble metals.
For commercial applications of shape memory alloys characterized by the omission of noble metals, generally only NiTi, CuZnAl and CuAlNi have been available.
NiTi shape memory alloys are known to have excellent properties. With an almost stoichiometric composition, they are characterized by a particularly high degree of reversible deformation with a one-way or two-way effect, by high tensile strength and ductility and by very good corrosion resistance. Moreover, when exposed to thermal cycling these shape memory alloys exhibit excellent stability of the magnitude of their shape memory effect. In addition, they can be heated relatively far beyond the temperature of the completion of austenite formation, A.sub.f, without the occurrence of damaging irreversible lattice changes which reduce the magnitude of the shape memory effect or inadvertently shift the transformation temperature.
To utilize the two-way effect, the temperature at which austenite formation begins, A.sub.s, should be relatively high, for example above 100.degree. C. However, the maximum attainable A.sub.s temperatures for NiTi shape memory alloys for repeated applications are below 100.degree. C.
Hereinafter the applicable A.sub.s temperature is considered to be that temperature which appears after several thermal cycles.
In the literature, the addition of zirconium as a third element in the place of titanium to raise the transformation temperature is disclosed. Eckelmeyer, in Scripta Met. 10 (1976), pages 667-672, discloses the effect of up to 2 atomic % Zr added instead of Ti. According to this article, the transformation temperature is raised by about 42.degree. C. per atomic % Zr. The highest A.sub.s temperature values measured lie at about 105.degree. C. for the one-way effect with 2 atomic % Zr; however, it was not clear whether A.sub.s, A.sub.f or a value therebetween was being measured. This publication does not consider alloys having greater Zr contents than 2 atomic percent.
Based on the above work Kleinherenbrink et al. examined and reported in The Martensitic Transformation in Science and Technology, given at a conference in Bochum, FRG, on Mar. 9-10, 1989, shape memory alloys including up to 1.5 atomic % Zr. No increased transformation temperature could be measured, that is, the result of the first-noted publication could not be confirmed.
At present, only shape memory alloys of a CuAlNi system are commercially applicable for repeated applications having an A.sub.s temperatures above 100.degree. C., as disclosed by Duerig Albrecht and Gessinger in A Shape Memory Alloy for High Temperature Applications, Journal of Metals 34 (1982), pages 14-20 . With these alloys, A.sub.s temperatures up to 175.degree. C. can be attained; however, these alloys exhibit significant drawbacks. For example, the maximum two-way effect is only 1.2%; elongation at rupture is low (5 to 7%), and tolerance of overheating is noticeably less than for NiTi shape memory alloys. Further, the low effect-stability is unfavorable for repeated applications: a significant decrease in the degree of reversible deformation occurs after only a few hundred temperature cycles.
In the past, no commercially usable shape memory alloy based on NiTi has been known which had an A.sub.s temperature of more than 100.degree. C., though the potentially favorable characteristics of such alloys has prompted the expenditure of considerable efforts.